Field of the Invention
The present invention concerns composite structures, especially multilayer ceramic capacitors, having properties of high thermo-mechanical strength.
In the field of multilayer ceramic substrates as well as in that of multilayer ceramic capacitors or of any other multilayer structure of the same type, the properties of resistance to thermal shocks are always of critical importance. Thus, in the technology of surface-mounted components (SMC technology), which is coming into increasing use, the process of high-temperature wave soldering (namely for a temperature equal to or greater than 260.degree. C.) imposes high thermo-mechanical stresses on the components to be soldered.
In the case of multilayer ceramic capacitors, there is no existing structure specifically adapted to the absorption of thermal shocks. Only the dimensions of the components are restrictive and enable the choice of the material or of the desired product to be directed as a function of its intended purpose. Depending on the formats of the components, one method or another will be used. Thus, for capacitors having formats of 0805 to 1210, it is possible to use flow soldering which imposes a high temperature gradient in the component. For capacitors having formats of 1812 to 2220 or more, reflow soldering must be used. This leads to a low temperature gradient in the component. For the latter formats, flow soldering would result in an excessive high reject rate. It will be recalled that the formats referred to are those of the international standards laid down by the EIA (Electronic International Association), and that the first two figures correspond to the length of the component in thousandths of an inch while the last two figures correspond to its width in thousands of an inch.
At present, in the field of multilayer SMC capacitors, there can be no question of mounting 2220-format components by a wave soldering process without a very high degree of pre-heating (the difference between the maximum temperature of the wave and the maximum temperature of the pre-heating is between 120.degree. C. and 130.degree. C.) which is designed to reduce the temperature gradient undergone by the component to below the temperature at fracture point.
In order to overcome the above-mentioned drawbacks, the present invention provides for the making, in a multilayer type composite structure, of zones with a particular distribution depending on the value of coefficient of thermal diffusivity of the materials used.